On this topic I will attempt to detail the building of a guitar amplifier in my shop.The 'brand' name if you will is 'Thermite' as I have plans to build more.The model of this amp will be the 'Hydra'.I expect the project to take at least a full year and cost over $2000.This thread will be locked for continuity's sake and anyone may start a seperate questions, comments, discussion thread at any time.

I chose this chassis for the quality. The powder coat finish is fantastic. The corners and seams are welded and ground so flush that you would swear the whole thing is molded. The screen printing also looks great. This was one of the most cost efficient decisions of the build, at a mere $138 US it was never worth building myself.

After a lot of research into many different brands of both OEM and after-market transformer manufacturers including Hammond, Heyboer, Mercury Magnetics and DeYoung Manufacturing, I decided to go with O'Netics transformers.Since these are the single most expensive part of any amplifier and have a huge impact on their sound, this is not where I wanted to cut any corners. Bud Purvine's transformers are a work of over engineered art.

This transformer is severely underated, it is capable of delivering up to 600mA on the B+ tap and is tested and guaranteed stable at 130° Celsius for up to an hour.

The output transformer is a 100W 'Hi-Def' model and has a primary of 2K2Ω and secondary taps of 4, 8 and 16Ω.It boasts a flat power bandwidth and passes transients quickly, a perfect match for a modern high gain amplifier.Again it is conservatively rated. Bud has assured me that it is safe for up to 300W and has the same thermal rating as the PT.

While on the subject of iron, the choke I chose was a 10H one from O'netics as well, it was designed specifically to be used in a tube amp.

The total cost for the three pieces was $512 USD plus taxes and shipping.

Package received.

First thing i did was to sand the laminates to prep them for painting.

Next using the end bells as masking I applied 5 coats of high temperature flat black paint.

I then removed the end bells and sand blasted them.

I chose to replace all the stock fastening hardware with all stainless steel barrel headed machine screws with Nylock nuts.

Chasing the threads after having to trim a few shorter.

I took the sandblasted end bells to Dragon Powdercoating and had them done in a light blue to follow the colour scheme of the build.

The end bells for the 10H Choke I decided to apply a baked enamel finish on them. Again they were sandblasted, 3 coats of primer, two coats of blue enamel and then baked in a rod oven @ 200°F for two hours.Here is the finished product.

Over the years I have replaced a good number of pots from budget models to mid priced stuff like Alps, Ohmeg and Panasonic to fairly rough service components like Bourns, Vishay and Allen-Bradley. I have replaced pots (rotary and linear) in mixers, stomp boxes, $4500 Avalon EQ's, and many a guitar amp. There is no real cure for a pot gone bad other than replacement.

I wanted the pots on this amplifier like all other components to last relatively forever. A few years ago I had purchased from a junkyard some electronic components out of a German tank and it hadn't gone un-noticed that the quality of the parts was way beyond the average MI fare.I ended up purchasing Clarostat RV4 series potentiometers, they comform to MIL-R-94.These are a conductive plastic, sealed, 500V, 2W, nickel plated brass shaft, stainless backshell, tinned lug, -65° to 125°, with a rotational lifespan of 25,000 turns.They are practically indestructible and could almost be used underwater.Total cost approximately $150. I know there are practice amps on the market that cost less then what I spent in pots alone!

The criteria here was to have a socket that would provide a tight connection to the pins that would not loosen over time.Another issue was some degree of mechanical isolation to reduce the chances of microphonics, this is often more of an issue in the first stage but is not to be discounted in the subsequent ones. It should go without saying that I wanted a socket that would incorporate a shield of some sort.I tried a few of the ones that I had on hand but decided to go with a set of blue anodized ones to follow suit on the colour scheme.For those who are not familliar, the most common type of tube socket these days for miniature preamp tubes including the popular dual triodes is a 9 pin arrangement called Noval.

Tube, socket and shield.

Notice the placement of a rubber 'O' ring.

From the under side we can see the rubber 'O' ring sandwiched betwixt the socket flange and the chassis.

Again here from the topside of the deck you can see how the socket is mechanically floated or decoupled from the chassis.

The orientation of the preamp tube sockets here is crucial, note that the secant from pins 4-5 to pin 9 is transverse to the long axis of the chassis, this will come into play later.

Most currently used beam output pentodes and kinkless tetrodes use what is commonly called an Octal socket.There were even instances in the past where these were used as cable connectors, in some cases they are still used for industrial control relays.

As with the Noval sockets the aim was the same regarding a sound connection. Whenever high voltages are present what is called the galvanic effect can be intensified, this occurs when you have dissimilar metals in contact with current passing through their junction. The by-product of this effect is often oxidation through electrolysis. I believe that this can be mitigated through firm mechanical contact. There are two main popular types of sockets for these, the semi-circular type, and the bifurcated type. I prefer the later.This is where I felt I needed to order samples for evaluation. Some of them were really cheap and some were pretty far out.I settled on a Belton brand Octal Micalex chassis mount socket with figure eight lugs.Spring type or bear trap type tube retainers will NOT be necessary with these.

Flange above or below deck?

I went with flange below deck. If you look carefully you can see the fork type socket.

Here is a shot from underneath, notice the figure '8' solder lugs, these are nice when tying in multiple conductors to the same pin. Also note the stainless steel hardware throughout, I am using hex drive machine screws and Nylock nuts, no slotted or Phillips drive.

With switches I got lucky.Switches can suffer similar woes as potentiometers, contacts can oxidize and or become contaminated.Starting with a sealed switch with either silver or gold plated contacts will help eliminate these issues.I was already a fan of C&K switches, I had used their 7000 series in the past and liked their construction. I contacted C&K's North American distributor and requested enough samples to do two amplifiers, they obliged. I specified all my needs and wants and got exactly what I wanted including locking actuators.

For the impedance selector switch I needed a rotary type, 90° travel, three position, break before make that would handle the current of the OT output, resist accidental throws and be reliable. A very long hunt led me to NKK a Japanese switch manufacturer. Their HS series provided me with the exact specs I required, they were a fair bit pricey, but again an email requesting two samples proved fruitful. They graciously shipped them immediately without even replying to my query.

C&K signal switch and NKK impedance selector switch.

Control switches installed.

Impedance control switch from the rear, installed.

For the mains and standby switches I went with Carling.I wanted to install a double pole switch for the mains, many amps use a single pole switch to open only the hot side of the mains. The problem with this is that if the mains wiring is somehow reversed 120V AC will still be present in the amplifier.I selected a 'G' series DPST for the mains, they are rated at 250V/15A.The standby switch is from the 'F' series and is rated for 10A. It is very difficult to find a toggle switch rated for >500V DC. With a high enough HP rating the switch should be able to withstand some arcing though.Fender® amplifiers have been using Carling 110 series for quite some time and I've not seen any fail, the 'F' and 'G' series have substantially better ratings then the 110 series.

Carling 'F' and 'G' series.

Mains and standby switches from the rear.

I decided to make use of the rectifier filament 5V winding for the pilot lamp. Since this amp will have a solid state rectifier this winding would have been otherwise unused.

I used vinyl tape to perform my layout, a fined tipped ball point pen, a small carpenter's square and an automatic center punch were all that was needed. Also step drilling for larger holes such as the grometted holes for the transformer leads is always a good idea.

Here you can see the stainless steel 3/8" circuit board standoffs which will support the preamp board.

Here are the holes for the 10 Henry choke which will be deck mounted. The red handled device is a de-burring tool.

Grounding.One of the main design goals of this amp was a high signal to noise ratio. The four main types of noise in this type of amplifier that need to be combated are Radio Frequency Interference (RFI), Electromagnetic Interference (EMI), thermal noise and hum.A proper grounding scheme will help reduce induced hum or 60Hz and its harmonics. I will be using a star ground on this build.

The first step is to tie the earth ground lug from the IEC power inlet directly to the chassis using a short wire. I used a star washer between the chassis and the lug terminal to ensure a sound electrical connection. I also used stainless steel hardware to reduce the chances of corrosion which could deteriorate the connection over time and increase resistance.

The second step is to mount another stud for the main or first star ground. At this point I tied the PT's shield ground (black wire w/ white stripe), the HT center tap (red wire w/ white stripe), the output section ground buss and the ground lead that will be going to the first filter ground on the power supply board.

At this point the output section ground buss terminates at the speaker out jacks' sleeve connection. I used two layers of polyolefin thermal shrink tubing to prevent the buss from ever accidentally shorting to the tip connection of the left jack. This is also where the OT's secondary common lead will tie in as well as the return path for the variable slave output.

This amp will include an unbalanced variable slave output.This feature can be useful to feed a separate power amp, effects unit, Direct Injection box or anything requiring a low level post EQ signal. To accomplish this I tied a 47k5Ω 1W resistor from the 8Ω OT tap which in turn gets connected to the wiper of a 5kΩ linear potentiometer. At a full 100 watts and the control fully CW this will present ~3V signal at the slave out. The tip connection of the 1/4" slave output jack gets tied to the pot and the sleeve connection, commoned to the speaker out sleeve connection.

This time I slipped a piece of clear heat shrink polyolefin tubing over the resistor to both protect the soldered connection and to allow a tech to read the device value.

Here you can see the .250" jack. It is fully insulated from the chassis to prevent ground loops. The tip connection is tied to the 5kΩ pot and the sleeve connection goes back to the left speaker out's sleeve tab.

The output transformer's secondary leads include 4, 8 and 16 Ohm taps which get connected to the NKK SP3T impedance selector switch. Grey is 16Ω, green is 8Ω and red is 4Ω. Notice the yellow wire from the slave out pot tying in with the green 8Ω lead. Also notice the brown small gauge wire tied to the 4Ω tap along with the red lead, this is the negative feedback going back to the preamp board via the depth (resonance) control.

The last lead from the OT secondary is the black one, it is the common or return line for the speakers, it also gets tied to the ground bus at the right hand speaker jack's sleeve connection tab.

The common or wiper connection of the impedance selector switch simply gets attached to the two tip connection tabs of the speaker jacks. These jacks are also fully insulated from the chassis by means of black nylon shouldered washers. You can see them behind the jack flange.

As for the primary of the OT, the center tap (white) ties back to B+ just ahead of the choke and the legs (blue/brown) to each pair of the tetrode's plates.

My O'netics power transformer has a filament winding supplying 3.15-3.15 VAC, there is no center tap for this winding.I created a faux center tap using a matched pair of 2 Watt 1% 100R resistors. Instead of tying the resistor junction to ground I will be using this faux center tap to elevate the heater reference by ~50V. This will be accomplished by adding a voltage divider from the plate supply right after the choke. By elevating the heaters I hope to reduce induced hum from the 60 cycle AC fed filaments. Also the f-K maximum differential is 100V-180V for 12AX7 type dual triodes depending on the model, this will certainly help keep me out of the danger zone.The PT has an ample 10 amps available at the heater winding. The KT77's spec shows that If=1.4A, 12AX7's in turn draw 150mA.The total current draw on the heater string is 6.35 A leaving lots of headroom.

The two green wires going to the two lug terminal strip are straight from the PT's 6.3 VAC winding. The twisted pair grey wires underneath run back to the preamp tube filament string. This is also where the output tube string will get its feed.

Here is the parallel heater run for the output KT's connected to pins 2 and 7.

Here is the parallel heater run for the preamp string using silvered buss wire. Remember the importance of the preamp tube socket orientation from my previous post, should be clear now.

No re-inventing of the wheel here. The class A/B push-pull power amplifier section will employ a long tail pair or 'Schmitt' phase inverter or PI, also known as a phase splitter. It is electronically categorized as a differential amplifier. A modern version of this is the everyday op-amp or operational amplifier.It was first introduced by Alan Blumlein, father of binaural sound and the famed Blumlein pair. Otto Schmitt developed the concept for amplifier use.This circuit has been used in guitar amps since the days of the Fender Bassman (5F6-A),the Marshalls Plexi (1987), JTM45, Bluesbreaker (1962), JCM800 (2203) etc. This drive circuit serves to divide the signal into an in-phase signal and an out of phase signal to drive the two pairs of output tubes into the balanced primary of the output transformer.

The negative feedback would tie in just below the tail resistor and feed the non-inverting input through the coupling capacitor, more on this in the next post.

This PA will employ a global negative feedback loop from the 4 ohm OT tap back in a the long tailed pair PI stage.Negative feedback in amplifier design is fairly common. Think of it as an input/output comparator. Typical amplifiers are not very linear and this helps to combat this trait.Firstly negative feedback helps straighten the frequency response, imagine a drop in low end response in the PA, the negative feedback loop then feeds the front end of the PA with low end deficient signal thus boosting those same frequencies at the output.I've included a filter for 'depth' or resonance control in the feedback loop. This is made up of a 4n7 film cap in parallel with a 1M log potentiometer. There will also conversely be a presence filter, since these are both in the FB loop, the idea is to cut the desired range in order to hear it's rise at the output. Keeping in mind that there can only be as much gain at any frequency as the net gain induced by the FB loop.NFB also helps lower distortion in the affected stage and reduces the output impedance of the stage. Reducing output impedance proportionately raises the damping factor which in turns increases the apparent tightness of the amplifier as it becomes less reactant.I plan on having a cut switch for the 4n7 bypass cap, the effect will be to divert the NFB through the 1M potentiometer and having control as to how much NFB is employed, this will be limited by a 47k series resistor.Some amps employ no NFB at all as there are some drawbacks, as discussed NFB gives an amp a tighter feel, less NFB conversely gives it a looser feel which may be desired. Another drawback of NFB is although distortion in the stage is reduced, when it is reached it is very immediate as the output stage begins to clip.

Class AB push pull using two pairs of KT77 kinkless tetrodes.Design again is classic and nothing new.

Supplant KT77's for the EL84's, and ignore pinout.

Add 1% 1Ω 3W resistors between cathode and ground to measure bias current.Given I=V/R, measuring the voltage drop across a 1Ω resistor gives you the current passing through it, no need to break the connection between ground and cathode. For and accurate measurement where a few milliamps has a large effect on the tube's dissipation the tolerance of this resistor must be tight.

Add 2k2 grid stop resistors @ g2.Grid risers.

Soldered to pins 5.

Heatshrink, my bias circuit theme colour is apparently yellow.

Bias feed resistors are 220k.

I used 630V 33nF coupling caps.

Screen resistors are 5W 1kΩ.I bussed the 497V post choke screen supply across the unused pins 6 of the octal sockets.I then soldered the 1k power resistors from pin 6 to pin 4.

First stage that the instrument signal sees is the B half of V1 (12AX7).The input signal shield is drained at the chassis end via the front panel ground bus which will be tied to the ground plane on the preamp board.The opposite end of the signal cable is terminated with a 68k resistor before connecting to the triode's grid.This resistor forms an RC filter when considered with the Miller capacitance of the tube, creating a low pass filter to deflect stray RF interference.Miller capacitance is a combination of factors including the grid to plate capacitance and the grid to cathode capacitance plus stray capacitances.

Typical 12AX7 values;

Cgk = 1.6pF + .7pF = 2.3pFCgp = 1.7pF + .7pF = 2.4pF

Miller capacitance formula is Cin = Cgk + Cgp * (A + 1), where A is the gain factor of the stage.Let's assume a gain factor of 74 for all frequencies, in reality gain is very frequency dependant.

I've made the effects loop half normalled by tying the send's tip connection to the normalling tab on the return's jack. This way one could use the send as a preamp out without breaking the signal path and conversely plug in to the return only, to make use of the PA with eq and feedback filters.

The effects return signal is buffered by V4B. Here you see the signal tying into V4's second grid at pin 7.

The return is grounded at the jack end which provides shielding drain.

Using a 1590 series cast aluminium enclosure from Hammond.Instead of using the typical permanently attached cable concept, I decided to simply mount a locking 1/4" TRS jack. If the cable ever gets damaged or fails it can be replaced instantly with a simple instrument cable.The reason for the diagonally opposed switch and pilot light is to allow for additional functions in the future without losing the symmetry.

The high density foam is to dampen the hollow ringy sound when depressing the footswitch.

I had the headshell custom built by a company in Texas.The vinyl covering is Regency Blue Bronco pattern from Kayline Processing Inc.I went with black hardware as well as opposed to the popular chrome.